My favorite workout toward getting this achievement is the Cardio Max. Despite the name, you can perform it in a pretty low-impact way. The trainer will tell you to add power, jumping whenever possible, but the Kinect isn't savvy enough to know if you're taking it easy.
To be honest, when you get to Set 2, you can walk back and forth for a while (with arm movements) and the sensor will still give you a five-star rating for those drills. You can get a final 5-star rating from this workout by giving minimal effort. Do this 19 more times over the course of a couple weeks, and this high-ratio achievement will easily be yours.
If you decide to give effort you can get about 32k-33k points per workout, pushing you in the right direction for the "A Cool Mil." achievement.